The invention concerns a method for producing nanostructures on a substrate. The present invention especially relates to the production of nanowires and/or linear arrangements of nanodots (clusters) as well as the production of carbon nanotubes directly on almost any desired carrier. The invention also concerns the production of nanostructures on silicon wafers, in particular on MEMS or microchips.
Nanostructures as well as nanowires and nanotubes are presently in the spotlight of current research. They represent a class of materials, which by virtue of quantum effects have novel electrical, optical, magnetic and thermodynamic properties, among other things. Aside from the primary academic questions, the problem of reproducible mass production with the simplest possible resources in order to promptly develop the industrial use of nanostructures is also presented.
Simple methods for producing nanoparticle aggregates are certainly known (refer to, for example, Tsapis et al, “Onset of Buckling in Drying Droplets of Colloidal Suspension,” Physical Review Letters, 94, 018302-1-018302-4, 2005); however, the production of extensive regular nanostructures on a substrate surface is to this day a difficult process, which is usually associated with several steps and high costs. Typical methods for the production of such structures are Vapor Liquid Solid (VLS) or MOCV methods. These methods are certainly relatively universally applicable, but atmospheric control (UHV), as well as also the need for high temperatures (600-1000° C.), require expensive equipment and make the synthesis time consuming. Prestructured substrates, such as, for example, MEMS, cannot be readily exposed to such high temperatures.
In order to reduce costs, the person skilled in the art is also acquainted with wet chemical production methods in aqueous solution, which show the desired results at temperatures below 100° C. and at atmospheric pressure (refer to, for example, Law et al, “Nanowire Dye-sensitized Solar Cells,” Nature Materials, 4, 455-459, 2005). However, apart from their very slow production flow (processing times of several hours to days), the wet chemical methods have other disadvantages. No epitactic growth is possible on silicon, for example (refer to J. Phys. Chem. B 2001, 105, 3350-3352). Solvents are used in many cases, which present problems with regard to their disposal.
There is great interest these days, for example, in the production of zinc oxide (ZnO) nanostructures, for example, nanorods and nanotubes. The reason is that, as a semiconductor, ZnO is able to form a large variety of nanostructures. In addition, its versatile applications as optoelectronic components, lasers, field emission and gas sensing materials are also taken into consideration (for the production and use of nanotubes and nanorods, refer also to Advanced Materials 2005, 17, 2477). For the epitactic production of ZnO structures are used either special substrates, such as gallium nitride (GaN), or silicon substrates coated with a so-called seeding layer, which generally consists of a ZnO thin film heated to 400° C. The direct non-epitactic production of extensive nanostructured ZnO structures on substrates is unknown to this date.
A further example are carbon nanotubes (CNT), which are called smart materials. They are used in fuel cells, biogas sensors, field effect transistors, among others. In this case, the known production methods (arc, laser, CVD, PECVD) also show a high technical complexity (high temperature, vacuum, etc.). An alternative solvothermal synthesis method at low temperature (310° C.) indeed solves this problem (Wang et al., Nanotechnology 16, 21-23, 2005), but the process still takes approx. 20-40 hours with a meager yield.
A third example is constituted by water soluble inorganic nanostructures, such as CaCO3, BaCO3, whose use is recommend for new applications in biotechnology due to their unusual mechanical and optical properties. The controlled production of such materials is carried out by mixing salts with polymers (biopolymers, refer to Shu et al, Nature Materials 4, 51, 2005). Wires made of such materials, which have a diameter of less than 100 nm, are unknown to this date.
An even more important example is the production of nanowires consisting of nanoclusters. It is already known that the arrangement of nanoclusters in 1D, 2D or 3D can lead to new properties, which are not present in disarrayed clusters, and which are based on the nearest neighbor interaction between the clusters, such as, for example, magnetization flip (application: data storage) and plasmon conductivity (application: optical fibers) (refer to as sources, for example: Nature Materials, 2, 229, 2003 or Eur. J. Inorg. Chem. 2455, 2001).
In contrast to the known and simple arrangement of nanoparticles in 2D and 3D, the arrangement in 1D is a complex procedure, with which a template (for example, mask, casting mold) must usually be used. This template limits the materials to be used, and may lead to disturbances in the formation of the nanowire from the clusters (when the template is removed, for instance) or its properties (for example, if a nanowire is used as a sensor, the template material that has not been entirely removed may reduce the sensitivity).
Hence, it is not unusual that the different processes, which are used for producing nanostructures, have common disadvantages, like complexity, high costs, low speed.